Assignees own U.S. Pat. No. 4,793,191 describes a mass flow sensor which can be installed, by means of flanges, in a conduit of a given diameter so as to be axially aligned with said conduit, through which flows a fluid to be measured,
with an inlet tube and an outlet tube, PA1 with an inlet manifold and an outlet manifold, PA1 with an external support tube PA1 with two annular diaphragms PA1 with two parallel, straight measuring tubes of the same inner diameter and the same wall thickness and each having its two ends fixed in the inlet manifold and outlet manifold, respectively, and PA1 with means which excite the measuring tubes into opposing resonance vibrations. PA1 which can be installed, by means of flanges, in a conduit of a given diameter through which flows a fluid to be measured, PA1 with a bent measuring tube extending between the flanges and traversed by the fluid, PA1 with a bent dummy tube extending parallel to the measuring tube and not traversed by the fluid, PA1 with an external supporting frame, and PA1 with means which act only on the measuring tube to excite resonance vibrations of the measuring tube. PA1 which can be installed, by means of flanges, in a conduit of a given diameter so as to be axially aligned with said conduit, through which flows a fluid to be measured, PA1 with a single straight, or substantially straight, measuring tube extending between the flanges and traversed by the fluid, PA1 with a support tube having its ends fixed to the respective flanges, PA1 with a compensation cylinder in which the measuring tube is fixed within the support tube and which does not touch the support tube, PA1 with means disposed between the measuring tube and the compensation cylinder for exciting resonance vibrations of the measuring tube, and PA1 with mass bodies mounted on the measuring tube to influence their natural frequency. PA1 which can be installed, by means of flanges, in a conduit of a given diameter so as to be axially aligned with said conduit, through which flows a fluid to be measured, PA1 with a straight measuring tube extending between the flanges and traversed by the fluid, PA1 with a straight dummy tube extending parallel to the measuring tube and not traversed by the fluid, PA1 with a nodal plate on the inlet side and a nodal plate on the outlet side, PA1 with a support tube having its ends fixed in the respective flanges, and PA1 with means which act only on the dummy tube to excite resonance vibrations of the measuring tube. PA1 with the electromagnetic system containing PA1 with the driver circuit generating an alternating current superposed on a direct current PA1 with the electromagnetic system containing PA1 with the driver circuit applying positive half waves of a driver signal to the first electromagnet, and negative half waves of said signal to the second electromagnet, PA1 with the electromagnetic system containing PA1 with the driver circuit applying positive half waves of a driver signal to the first electromagnet, and negative half waves of said signal to the second electromagnet,
which serve to connect the mass flow sensor with the conduit, PA2 whose ends are fixed to the inlet tube and outlet tube, respectively, PA2 via which the inlet and outlet tubes are connected to the inlet manifold and outlet manifold, respectively, PA2 with the measuring tube and the dummy tube clamped in an internal supporting frame, PA2 one of which fixes the inlet-side end portion of the measuring tube to the corresponding end portion of the dummy tube, and PA2 the other of which fixes the outlet-side end portion of the measuring tube to the corresponding end portion of the dummy tube, so that the measuring tube and the dummy tube are arranged side by side, PA2 a sleeve of soft magnetic material surrounding the dummy tube, PA2 a first electromagnet with a first U-shaped core and a first coil, and PA2 a second electromagnet with a second U-shaped core and a second coil, PA2 by applying a signal representative of the vibrations of the measuring tube to the first input of a phase comparator, PA2 by applying a signal generated from a signal representative of the vibrations of the dummy tube by means of an adjustable phase shifter to the second input of the phase comparator, and PA2 by integrating the output of the phase comparator, PA2 the alternating current being adjusted to the resonance frequency of the measuring tube by means of a phase-locked loop. PA2 a sleeve of soft magnetic material surrounding the dummy tube, PA2 a first electromagnet with a first U-shaped core and a first coil, and PA2 a second electromagnet with a second U-shaped core and a second coil, PA2 the electromagnets being located diametrically opposite to each other with respect to the sleeve, and PA2 the driver signal being the output signal of a phase shifter PA2 a sleeve of soft magnetic mmaterial surrounding the dummy tube, PA2 a first electromagnet with a first U-shaped core and a first coil, and PA2 a second electromagnet with a second U-shaped core and a second coil, and PA2 the driver signal being the output signal of a voltage-controlled oscillator,
Furthermore, assignees own U.S. Pat. No. 4,949,583 describes a mass flow sensor with a single straight measuring tube which is excited into peristaltic vibrations of its cross-sectional area.
Mass flow sensors working on the above-mentioned principle with two straight measuring tubes vibrating in the manner of strings have proved to be effective in practice. However, for various reasons, e.g., because of the sensitivity of the mass flow sensor to vibrations stemming from the conduit or because of the dependence of the measurement result on the pressure of the fluid, the diaphragms cannot be made arbitrarily soft but must have a given minimum stiffness, so that the above influences cannot be completely suppressed.
Since, in addition, changes in the temperature of the fluid result in nonhomogeneous temperature distributions in the mass flow sensor, stress is caused in the vibrating straight measuring tubes and in the diaphragms. If this stress reaches values above the yield point of the diaphragms, irreversible plastic deformations are caused which irreversibly change the characteristics of the vibrating system, so that recalibration of the mass flow sensor will become necessary.
As further prior art shows, experts have been working on a solution to these problems for a long time. EP-A 473 919, for example, describes a mass flow sensor working on the Coriolis principle and forming part of a dosing mechanism
In this prior art mass flow sensor, the dummy tube serves as an antiresonantor, so that a tuned internal vibrating system consisting of measuring tube, antiresonator, and internal supporting frame is obtained. Its dimensions are calculated in advance by the finite-element method. Since, however, the resonance characteristics of the vibrating system depend primarily on the type of the fluid and also on the density and current temperature of the fluid, it is clear that a separate calculation has to be performed for each fluid, so that different results and, thus, different dimensions are obtained. For universal mass flow sensors which are to measure many different kinds of fluids, the proposal of EP-A 473 919 is therefore practically unsuitable.
DE-A 41 43 361 describes a mass flow sensor working on the Coriolis principle
In this prior art mass flow sensor, the measuring tube is so installed in the compensation cylinder as to be under tensile stress at the normal temperature of the fluid. As the temperature rises, the tensile stress decreases due to the different expansion coefficients of compensation cylinder and measuring tube, and as the temperature rises further, the tensile stress turns into compressive stress. The upper temperature limit is therefore increased as compared with a measuring tube without tensile prestress.
Starting from U.S. Pat. No. 4,793,191, the first of the above references, it is an object of the invention to provide a mass flow sensor which is fitted with two straight tubes, but in which the manifolds and, thus, the diaphragms can be dispensed with and in which no compensating mass bodies are necessary on the measuring tube. Nevertheless, as far as possible, no vibrations are to be transmitted from the measuring tube to the support tube. Thus, a property of the mass flow sensor of U.S. Pat. No. 4,793,191, which is based there on the use of the diaphragms, is to be retained, but it is to be achieved in a different manner.